1. Field of the Invention
The present invention relates generally to digital waveform generating apparatus, and is directed more particularly to a digital waveform generating apparatus suitable for use, for example, to generate audio waveforms in an electronic musical instrument.
2. Description of the Prior Art
In a prior art analog electronic musical instrument, for example, a music synthesizer, various waveforms, such as a sinusoidal, triangular, saw-tooth, or other waveform are generated by means of one or more voltage controlled oscillators (VCOs). In the prior art, however, since the audio signals are generated using analog techniques, various problems arise in the accuracy of frequency, stability, and freedom of selecting generated waveforms.
Recently, in order to avoid the above-mentioned problems, attempts have been made to generate audio signals digitally.
The advantages of a system that digitally processes an audio signal to generate a desired waveform are that a rather high stability of frequency is obtained, any arbitrary desired waveform can be easily generated and, in addition, time division superimposing of various audio signals is possible, whereas such time division superimposing is difficult in an analog system. Further, is is simple to control a digital system and also the digital system can readily memorize a generated sound. Owing to the last-mentioned advantage of the digital system, it is possible to generate a sound that is remarkably similar to a natural sound by using a number of digital sound sources.
In general, a system to generate a waveform by digital techniques includes an ROM (read only memory), RAM (random access memory) or shift register to store, as digital data a desired waveform or such a value that is obtained by sampling one period or a predetermined number of periods of a fundamental waveform, and the stored digital data is used to generate a desired waveform by synthesizing, at a sampling frequency higher at least twice as high as the highest frequency contained in the fundamental waveform. Basically, two types of such digital systems may be considered. A first type of digital waveform generating system is a so-called variable clock system, in which the stored waveform data are sequentially read out by a clock whose speed is varied in correspondence with a selected musical scale frequency to produce a musical sound with that musical scale frequency. A second type of system is a so-called fixed clock system, in which an address signal varies the separation between successive addresses of the waveform data to an amount that corresponds with the musical scale frequency applied to the ROM or RAM to produce a musical sound with that musical scale frequency. In the case of the variable clock system, when such system is constructed to be able to sequentially vary the frequency, it is rather difficult for the system to have good frequency stability.